For example, a bias T circuit is disposed between a signal line through which a signal passes and a power supply, and superimposes a direct-current component (a direct-current voltage or a direct current) upon the signal line. There is a bias T circuit configured to prevent a signal from entering a power supply line from a signal line, in particular, prevent a plurality of signals in different frequency bands from entering the power supply line.
For example, Japanese Unexamined Patent Application Publication No. 2010-232988 discloses a wide-band bias circuit configured to supply a direct bias current to an amplification circuit that uses a predetermined frequency band and amplifies a wide-band high-frequency signal. This wide-band bias circuit includes inductors at three or more stages connected between a power supply and at least either of nodes on input and output sides of the amplification circuit so as to achieve a high isolation from the power supply in a wide band.
Japanese Unexamined Patent Application Publication No. 2011-82809 discloses a bias circuit configured to supply a direct-current component to an alternating-current signal input into an active element such as a transistor. This bias circuit includes a bias supply terminal for supplying a direct-current component to an active element, a parallel capacitor connected between the bias supply terminal and the ground, and a parallel circuit that is connected in parallel to the parallel capacitor and has one end connected to the bias supply terminal so as to supply a direct-current component to the active element in a plurality of frequency bands at the same time. This parallel circuit includes N inductors connected between a direct-current power supply node and the bias supply terminal and (N−1) series resonators (capacitors and inductors) each connected between the ground and a node between adjacent ones of the inductors.
In current years, there are electronic apparatuses such as personal computers that perform radio communication using a plurality of different radio communication methods (for example, the Wi-Fi (registered trademark) method and the WiGig (registered trademark) method). In the Wi-Fi method, radio communication is performed using a 2.4 GHz band and/or a GHz band. In the WiGig method, radio communication is performed using a 60 GHz band. A radio communication unit employing the WiGig method (a WiGig radio communication unit) generates a 60 GHz-band signal (millimeter-wave signal) by combining signals in three different frequency bands.
In a case where an electronic apparatus includes a plurality of radio communication units employing different radio communication methods (in particular, in a case where these radio communication units are disposed close to each other), noise in a predetermined frequency band which has been caused by the radio communication unit employing one of the radio communication methods may cause the degradation in communication quality of the radio communication unit using another of the radio communication methods (may cause self-poisoning). For example, in a case where 2.4 GHz-band noise or 5 GHz-band noise occurs in a WiGig radio communication unit, the noise may adversely affect the radio communication of a radio communication unit employing the Wi-Fi method (a Wi-Fi radio communication unit) using this frequency band.
Therefore, in a case where a radio communication unit employing a certain radio communication method may cause noise in a frequency band that is used in another radio communication method, it is necessary to remove the noise in this frequency band from a signal line because the noise may be emitted from a coaxial cable. It is also necessary to prevent the noise in this frequency band from entering a power supply line from the signal line via a bias T circuit.
Japanese Unexamined Patent Application Publication No. 2010-232988 discloses a technique for preventing a plurality of high-frequency signals in different frequency bands from entering a power supply line from a signal line by adjusting the inductances of inductors connected in series in a bias circuit to increase impedances in the frequency bands of the high-frequency signals. However, since a filter or the like for removing noise in a frequency band other than the frequency bands of the high-frequency signals (for example, the above-described noise in a frequency band used by another radio communication method) is not disposed in the bias circuit, it is impossible to remove the noise in this frequency band from the signal line and prevent the noise in this frequency band from entering the power supply line from the signal line using the technique disclosed in Japanese Unexamined Patent Application Publication No. 2010-232988.
Japanese Unexamined Patent Application Publication No. 2011-82809 discloses a technique for preventing a plurality of alternating-current signals in different frequency bands from entering a power supply line from a signal line by adjusting the inductances of inductors connected in series in a bias circuit and the resonant frequency of each series resonator connected between the corresponding inductors to increase impedances in the frequency bands of the alternating-current signals. However, since an inductor is present between the signal line and the series resonator including an inductor and a capacitor, a resonant frequency viewed from the signal line and a resonant frequency viewed from the power supply line differ from each other under the influence of this inductor. It is therefore impossible to remove noise in a frequency band other than the frequency bands of the alternating-current signals (for example, the above-described noise in a frequency band used by another radio communication method) from the signal line and prevent the noise from entering the power supply line from the signal line using the technique disclosed in Japanese Unexamined Patent Application Publication No. 2011-82809.
It is an object of the present disclosure to provide a bias T circuit that is disposed in one of a plurality of radio communication units included in an electronic apparatus and is capable of removing noise in a frequency band used by another of the radio communication units from a signal line and preventing the noise from entering a power supply line from the signal line.